1. Field of the Invention
This invention relates to digital-to-analog converters (so-called DACs). More particularly, this invention relates to high-resolution converters, e.g. for converting 16-bit digital input signals into corresponding analog signals.
2. Description of the Prior Art
There has been a growing need in recent years for high-resolution digital-to-analog converters. The resolution of known types of monolithic converters employing R/2R ladder seems to have reached an apparent practical limit of about 12 bits. Thus it has become necessary to look for other solutions to the problem.
It has been recognized that in some applications, absolute accuracy in digital-to-analog converters is less important than good differential linearity and guaranteed monotonic behavior. Superior performance in such respects can be achieved by segment-type DACs comprising strings of series-connected resistors with switches to make connection to selected nodal points of the string.
Segment converters can be arranged in cascaded format, such that a first stage employing a resistor-string converter decodes a group of higher-order bits, and a second stage decodes the remaining, lower-order bits. A non-linear converter of that general type is shown in the article by Gryzbowski et al., "Non-linear Functions from D/A Converters", Electronic Engineering 1971, pp. 48-51. The converter disclosed in that article is designed for operation with relay switching, not modern semiconductor switching. U.S. Pat. No. 3,997,892 (Susset) shows a cascaded (non-linear) converter design intended for use with semiconductor switching. In this design, both the first and second stage comprise a resistor-string segment-type converter. The converter design includes buffer amplifiers to prevent the second-stage resistor string from loading the first-stage resistor string.
Although the prior art converters referred to above embody certain attractive aspects, they are not capable of the high-resolution performance now required in many applications. Thus it is an object of this invention to overcome the deficiencies of such prior art schemes.